PROJECT5PROJECTSUMMARY Human cytomegalovirus (HCMV) remains a significant cause of morbidity and mortality after Hematopoietic Stem Cell Transplantion (HSCT) and Solid Organ Transplantation (SOT). A commonly encountered clinical manifestation of HCMV infection in the HSCT or SOT recipient is myelosuppression. HCMV reactivation and use of the antiviral ganciclovir is associated with a number of cytopenias that increase the risk of secondary bacterial or fungal infections and require growth factor support or transfusion of blood products. Despite the clear clinical relevance of myelosuppression to the transplant recipient with HCMV infection, little is known about the mechanism(s) by which HCMV infection inhibits normal hematopoiesis. HCMV has been shown to inhibit hematopoiesis by the direct infection of hematopoietic progenitor cells (HPCs) and indirectly by the effectofinfectedHPCsonthemicroenvironmentsupportinghematopoiesis. We have recapitulated HCMV myelosuppression in vivo using a humanized mouse model. We furthershowinbothinvitroandinvivomodelsthattheadditionofincreasingnumbersofinfectedCD34+HPCs increases the degree of myelosuppression and that supernatant from infected CD34+ HPCs, as opposed to mock-infected HPCs, suppresses myelopoiesis. Further, Projects 1, 2, 3 and 4 have identified viral proteins andmiRNAsthatdirectlyaltersignalingandeitherpromoteorsuppresshematopoiesisinvitro.Therefore,we hypothesize that HCMV infection reprograms signaling and cytokine secretion in infected cells resulting in a microenvironmentthatinhibitshematopoiesisandcontributestoclinicalmyelosuppressionandhematopoietic failure.Totestourhypothesis,weproposeasystemsapproachtodefinethechangesinsignalingandcytokine secretionusingbothinvitroandinvivomodels.Weproposethefollowingspecificaims.SpecificAim1.How doesHCMVinfectionofCD34+HPCsinvitroregulatehematopoiesis?WehypothesizethatHCMVsuppresses CD34+ HPC differentiation by altering signaling and secretion in the infected cell. Specific Aim 2. How does HCMV regulate hematopoiesis in vivo? We hypothesize that HCMV infection alters the microenvironment for hematopoiesisinthehostorganism.SpecificAim3.WhatarethemolecularsignaturesassociatedwithHCMV myelosuppression in HSCT and SOT patients? We hypothesize that HCMV-mediated changes in cytokine secretionimpedehematopoieticreconstitutioninHSCTandSOTpatients. IMPACT:Takentogether,ourprojectprovidesauniqueopportunitytomakeunprecedentedadvancements in our understanding of the mechanistic basis of HCMV-mediated myelosuppression. This advancement is driven by the development of the state-of-the-art huBLT mouse model and the identification of virus-host interactionsimpactinghematopoiesisandthekeycytokineandmolecularsignaturesthatpredictdisease.